1. Field of the Invention
The present invention relates to stratified structures for bonding plastic materials.
2. Description of the Prior Art
In recent years, the greater understanding of structure-property relationships of plastic materials and the introduction of new polymerization techniques and monomers have resulted in the availability of a large number of different types of plastics having a wide variety of properties. Plastic materials can be produced with the ideal properties necessary for a specified application. For instance, plastics may be hard or soft; rigid or flexible, rubbery, leathery, or glassy; porous or non porous; transparent or pigmented; thermosetting or thermoplastic, etc. These varying properties result from differences in chemical composition, atomic arrangement, molecular weight, and other structural aspects of the plastics.
In many instances, plastics having diverse properties are incorporated in a single plastic article. For example, a plastic package may include a transparent rigid window portion formed of polymethacrylate mounted on an opaque, tough housing of reclaimed non transparent polyvinyl chloride. The latter material is low in cost but does not offer the necessary transparent qualities.
In another application, it may be desired to provide fasteners on a polypropylene sheet. While the fasteners may be injection molded during the formation of the sheet, such as process leaves a dent on the side of the sheet opposite the fastener. Further, a fastener formed of polypropylene may not have the necessary mechanical strength. It is therefore preferable to affix a nylon fastener to the polypropylene sheet.
In other applications, parts of an article may be porous while other parts are non porous. The fabrication of such articles may be accomplished by numerous, available techniques, such as the use of adhesives, mechanical fasteners, and the like.
However, it is often difficult to find an adhesive which will bond article portions of dissimilar property plastics. The use of adhesives may lead to the contamination of the contents of plastic container articles. With mechanical fasteners, hermetic bonds may be impossible to obtain.
A preferable method of assembling plastic articles utilizes the thermal properties of the plastics to effect joinder. For example, the abutting portions of two plastic article portions may be heated to fusion temperature and then pressed together to effect a heat seal between the portions.
However, the differing structure and properties of the plastics often cause a greater or lesser degree of thermal bonding incompatibility between the materials which prevents formation of a satisfactory heat seal. This phenomenon has proven particularly troublesome in the fabrication of articles having portions of differing plastic materials and is proving to be an even greater impediment to fabrication as the number of diverse property and structure plastics continues to increase. Further, in cases in which a usable heat seal can be effected, this seal may be subsequently rendered ineffectual by conditions to which the article is exposed in use, such as oxidizing or temperature conditions.